Nonmagnetic armor



Patented Sept. 14, 1948 p IrviniB. Kramer, W'aShingtonQ DLQ.

Serial No. '2

1 Claim. (Cl. 14831) (Granted under the act of .Marchfi, 1883, 5 5 mended April so, 1923;; 370.0 9. v157) No Drawing. Application January zfi, 1 245,

My invention relates to steel alloys Suitable for tained by adding large amounts pf chromium, i 'ar Qr plate and, particularly, tononl-finagnickel or other alloying elements suchas mangahardflductile productsand the method'of re nese which elements havethe effect 9i inhibiting the alloy composition to'i jheat'treatment ithe austenite-pearlite transformation. "t impart "to the alloy the desired properties of 5 'Austenitic steels although they are usually nonhardness, ductility and magnetic permeability. magnetic, and probably stainless, aregenerally fe ntio 's to producea steel soft. They can be hardened, but only at the qost y r p t-lg; stic "ropelrties at of a considerable, lossfinjductility; y'lfhus armor least asgiiodas'" yof"thealloysus'ed as armor plate generally represents a comprqmisebetween and in addition thereto to have the quality of be- 10 composition and attainable physical proporties. ing non-magnetic. Since extreme hardness and great ductility are Another object of my invention is to provide the prime essentials of good armor plate, all else a method of controlling the composition of the is sacrificed in the attainment of ultimate values alloy to render it capable of developing the desired of these two properties, very often by Way of proper-ties and, further, to provide a sequence of elaborately expensive alloying elements.

treating steps which will impart to it the hard- I have found that common alloy steels can be ness and ductility required of good armor plate converted into extremely hard, highly ductile, yet will permit it to retain its non-magnetic qualnon-magnetic and non-magnetizable products ity. possessing ballistic properties at least as good as In the pas-t it has generally been accepted that acceptable armor plate is the manganese content a steel possessing a limit velocity of 1450 feet of the steel is balanced against the carbon content per second when 0.25 inch armor plate is tested so that after cold working to the desired hardness with thirty caliber bullets represents a satisfacthe steel can be maintained at a temperature high tory combination of physical properties to make enough to relieve the cold work stresses and imthe material acceptable for use as armor plate. part ductility to the steel and yet not cause the It has been found that such steels, because of precipitation of carbides from solid solution. The

their magnetic qualities, when used as armor method of carrying out my invention and producplate in aircraft necessitate the removal of the ing the alloy having the desired properties will be compass to a wingtip in order to minimize effects readily understood from the following examples: on its behavoir. The complications involved in 0. Six samples of an iron alloy containing 0.90% the installation of remote compass reading equipcarbon, 11% manganese, 0.60% silicon and 2.5% ment weigh heavily against the use of armor nickel were prepared and cold rolled to Brinnell plate in aircraft. hardnesses, respectively, of 294(1) 341(2) My invention comprises the several steps and 415(3), 363(4), 363(5), and 363(6). the relation of one or more of such steps with re- All of the samples were then given heat treatspect to each of the Others Which Will be exemments to relieve internal stresses as summarized Plified in the treating process hereinafter in the following table. The samplesl 2' 3 4 and closed necessary to be carried out to produce said 5 after heat treatment were and alloy, and the scope of my invention will be innon magnetizable dicated in the claim.

It is well known in the metallurgical art that o- Balhstm tests were performed on these Samples austemte is usually nommagnefia and the results determined in terms of limit venetic steels having austenitic structures are relaloclty 0f thespeclmen' Llmlt Velomty 1S defined tively common and are exemplified generally by as the velocity of the bullet necessary to penethe stainless steels. The austenitic structure of irate a p ate Such that the base of t e bu t ust known steels and, hence, the stainless quality p s s u h th p at Th nature of the and, incidentally, the non-magnetic quality, is difracture of the plate around the nose of the bullet rectly traceable to the identity of the particular when described in terms of the condition of the alloying elements and their proportions; The petals gives an indication of the ductility and austenitic structure in the stainless steels is obsuitability of the material as armor plate.

The following table summarizes the results of the ballistic tests:

t l Metallographic examination revealed heavy carbide precipita- From the data summarized in the above examples and in the table, it is apparent that a nonmagnetic armor having excellent ballistic properties can be produced by so balancing the manganese and carbon contents of the steel that the carbide precipitation temperature of the alloy will lie above its stress relieving temperature.

Since many changes may be made in the composition of the armor plate and thus many embodiments of the invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

' The invention described herein may be manufactured and used by or for the Government of the United States of America for government purposes without the payment of any royalty thereon or therefor.

Having described my invention what I claim as new and desire to secure by Letters Patent of the United States is:

A hard, ductile, non-magnetic steel armor plate consisting substantially of 11 percent manganese, 2.5 percent nickel, 0.60 percent silicon, and 0.90 percent carbon, the balance being iron and small quantities of residual elements commonly occurring as impurities in steel, made by the method, comprising cold-rolling the austenitic plate to a hardness of 363 Brinnell and heating the coldrolled plate at a temperature of 450 C. to relieve internal stresses.

IRVIN R. KRAMER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,732,202 Hall Oct. 15,1929 2,358,799 Franks Sept. 26, 1944 2,378,991 Franks June 26, 1945 2,378,993 Franks W June 26, 1945 2,378,994 Franks June 26, 1945 OTHER REFERENCES I High Manganese Austenitic Steels, Franks et a1, reprint from Iron Age October 1, 1942. 

